A belt-type continuously variable transmission is known in the art as a transmission mechanism of a vehicle. The belt-type continuously variable transmission comprises: a primary pulley of a driving side or an input side; a secondary pulley of a driven side or an output side; and a belt applied to those pulleys. In the belt-type continuously variable transmission, a sped change ratio is set acceding to a running radius of the belt thus applied to those pulleys. Each pulley comprises a fixed sheave and a movable sheave, and a groove between those sheaves to which the belt is applied is changed to vary the running radius of the belt. Specifically, the fixed sheave is integrated with a rotational shaft, and the movable sheave is adapted to reciprocate axially on the rotational shaft thereby varying a clearance between those sheaves. In addition, opposed faces of those sheaves are individually tapered to form a groove between those sheaves and the belt is applied the groove thus formed.
A transmission torque capacity of the belt-type continuously variable transmission is varied in accordance with a contact pressure between the belt and the pulleys, that is, in order to transmit the torque, the belt has to be clamped by pushing the movable sheave toward the fixed sheave. For this purpose, it is necessary to carry out a control for positioning the movable sheave to set a speed change ratio, and a control for clamping the belt by pushing the movable sheave toward the fixed sheave simultaneously in the belt-type continuously variable transmission. Therefore, in the prior art, the speed change ratio is controlled by controlling a position of the movable sheave of any one of the pulleys (e.g., the primary pulley of the driving side), and the speed change ratio is controlled by controlling a pressure pushing the movable sheave of the other pulley (e.g., the secondary pulley of the driven side).
Specifically, the speed change ratio is controlled by carrying out a feedback control of a flow rate of pressure fluid such as operating oil to be fed to an actuator of the primary pulley. However, even in case of keeping the speed change ratio constant, the pressure for clamping the belt is still required. Therefore, in order not to vary the position of the movable sheave, the pressure fluid is being fed to the movable sheave in a slight amount. Indeed, as a result of thus feeding the pressure fluid continuously without carrying out a speed change, a leakage of the pressure fluid is also continued and this causes an energy loss. For example, in order to avoid such a disadvantage, Japanese Patent Laid-Opens Nos. 2006-300270 and 2007-57033 disclose an improved belt-type continuously variable transmission. According to the teachings of Japanese Patent Laid-Opens Nos. 2006-300270 and 2007-57033, a control valve (or one-way valve) adapted to open only to a feeding side of operating oil is arranged on an oil passage for feeding the operating oil to push the movable sheave toward the fixed sheave, and a control valve (or one-way valve) adapted to open only to a discharging side of operating oil is arranged on an oil passage for discharging the operating oil to move the movable sheave away from the fixed sheave. The belt-type continuously variable transmission taught by Japanese Patent Laid-Opens Nos. 2006-300270 and 2007-57033 further provided with a hydraulic system that selectively prevent the control valve from opening.
In addition, Japanese Patent Laid-Open No. 5-33839 discloses a mechanism for controlling a centrifugal pressure, and Japanese Patent Laid-Open No. 2005-273730 teaches to divide a hydraulic chamber for setting a clamping force into two chambers having different pressure receiving areas, and to determine a ratio between those pressure receiving areas.
In a vehicle, a speed change ratio of high speed side is set comparatively frequently. In case of keeping the speed change ratio of high speed side, it is possible to reduce energy loss by preventing or reducing a leakage of the operating oil by preventing discharging of the operating oil using the hydraulic system taught by Japanese Patent Laid-Opens Nos. 2006-300270 and 2007-57033. However, a control for selectively stopping the operating oil being discharged to carry out a downshifting is basically executed electrically. For this purpose, the aforementioned hydraulic system is provided with a solenoid valve controlled electrically to selectively prevent the control valve of the discharging side from opening. However, if an electrical failure occurs, the control valve of the discharging side is kept to be closed, and as a result, the speed change ratio may be fixed to the high speed side. The belt-type continuously variable transmission is configured to maintain a torque transmission capacity by clamping a belt by a pulley, therefore, the speed change ratio cannot be varied if the transmission is not rotated. For this reason, if the vehicle is stopped under the situation where the speed change ratio is fixed to the high speed side, a driving torque may be insufficient to restart the vehicle. As a result, in the worst case, the vehicle cannot be restarted, or otherwise, the engine stall may occur.